Lithostratigraphy
In the Mt. Mucrone region the EMC consists of metagranitoids, mainly derived from Permian intrusives, and metasediments (mainly paragneisses and micaschists), descending from high-grade paragneisses that represent the country rocks of Permian intrusives, with minor metabasics.
The top of Mt. Mucrone, in the north-western sector of the studied area (Fig.2), mainly consists of Permian metagranitoids in primary contact with the hosts rocks (Maffeo, 1970; Oberhaensli et al., 1985). Part of metagranitoids suffered only Alpine metamorphism, preserving the igneous structure (grey-type metagranitoids), whereas the others recorded also an intense Alpine deformation (green-type metagranitoids). The grey-type metagranites represent a small volume (< 20%) of the whole igneous Permian complex.
Figure 2. Structural map of the southern slope of Mt. Mucrone with foliation trajectories traced on standard lithological information.
Relative chronology of superposed foliations (S1, S2, S3, etc.) is graphically represented by different colors of dashed lines. The inferred metamorphic conditions, under which successive foliations, folds and shear zones developed, can be inferred by the supporting mineral assemblages listed in the legend.
In the grey-type metagranitoids hypidiomorphic-granular primary textures are still recognizable, as well as compositional and grain size heterogeneities inherited from the igneous protoliths, consisting of: granites, minor quartzdiorites, pegmatites and aplites. In the poorly-deformed volumes the igneous contacts are generally preserved. Grey-type metagranites had a primary igneous composition consisting of: Qz (40%), Pl (35%), Kfs (15%) and Bt (10%). Only Bt, Kfs and rare Aln are partially preserved, while the Pl micro-site is totally replaced by Qz, Jd and Zo; Ph and Grt developed in coronas between Bt, Pl and Kfs. The grain size is medium and the texture is weakly porphyritic and the Bt content is ≤ 10%. Metaquartzdiorites generally occur in metre-thick boudins inside the grey-type metagranites; the main outcrop, a hundred meters wide, is embodied within the paragneisses (Fig. 2). This rock type is medium- to coarse-grained, with isotropic texture; the igneous structure is poorly preserved due to the widespread replacement of igneous phases by Alpine minerals: Omp (35%), Grt (25%), Qz (20%), white mica (10%) and Gln (10%). The inferred igneous modal composition with Pl (30%), Qz (20%), Kfs (10%) and mafic minerals (40%) is very close to that of a quartzdiorite or quartztonalite. Metaaplites and metapegmatites are fine-grained and coarse-grained rocks, respectively, with Jd, Grt and white mica and occur as centimetre to metre thick layers.
Green-type metagranitoids are fine- to medium-grained rocks with gneissic texture and a pervasive foliation underlined by white mica and clinopyroxene shape preferred orientations (SPO). These green coloured metagranitoids constituting about 80% of the Mt. Mucrone intrusives contain Jd or Omp, Grt, white mica and Gln. They do not show the same primary textural and compositional heterogeneities displayed by the grey-type metagranitoids, due to the strong textural and metamorphic reworking. These metagranitoids have been interpreted as a more deformed and mineralogically re-equilibrated (under eclogite facies conditions) equivalent of the “grey-type” ones (Castelli et al., 1994). The most representative mineral association of the green-type metagranitoids is: Qz (30-50%), Na-Cpx (20-30%), white mica (15-20%), Grt (15-20%), Zo (5%) and Gln (5%). Leucocratic metagranitoids are exposed at the south-western margin of the mapped area and they do not show primary or deformed contact with the other metagranitoids. Generally they have a gneissic texture, without significant textural and compositional variations, and are medium- to fine-grained with a penetrative foliation marked by white mica SPO and Qz-ribbons. The mineralogical composition is: Qz (40%), Ab (40%) and white mica (20%).
The metasediments consist of paragneisses and micaschists with interlayered minor metapegmatites, glaucophanites and zoisitites.
Paragneisses are medium-grained and show a mineralogical foliation marked by alternating Qz and Grt-layers, with Omp-Grt- and Gln-layers, parallelised to S1.
Metapegmatites (porphyric gneisses in the legend of Fig. 2) occur in metre-thick layers within paragneisses and contain coarse-grained Kfs porphyroclasts (20%), Qz (35%), Ab (25%) and white mica (20%). As already suggested in adjacent areas, they are interpreted as deformed and eclogitised leucosomes (Zucali, 2002; Zucali et al., 2002b), deriving from pre-Alpine partial melting of the high grade gneisses (kinzigites). The dominant foliation in these gneisses is defined by white mica SPO and high strain of quartz-feldspar-bearing domains.
Micaschists are fine-grained rocks containing white mica, Qz, Grt, Gln and Jd or Omp. The dominant foliation (S1 in paragneiss and S2 in micaschists) is marked by alternating quartz- and mica-rich layers and has millimetre scale spacing. The boundary with the paragneisses is transitional, due to the large amount of mica in the highly-deformed paragneisses grading into the micaschists.
Metre to ten-metre thick glaucophanites occur in lenses or layers at the boundaries between micaschists and metagranitoids. They show a spaced foliation, marked by Gln SPO and cut by randomly oriented Omp porphyroblasts.
Zoisitites occur as ten-metre thick enclaves in the green-type metagranitoids and at the margin of the green-type metagranitoids; they contain ten centimetres-sized garnets, locally showing compositional zoning. The mineral mode of zoisitites is: Zo (35%), Qz (15%), white mica (10%), Omp (10%) and Grt porphyroblasts (30%). Ten-centimetre to metre-thick Qz-rich veins occur within metagranitoids and their country-rocks and contain minor amounts of white mica, Grt, Gln and Zo. Metabasics are medium- to fine-grained eclogites, locally showing a S2 discontinuous foliation, marked by Amp SPO. They are crosscut by millimetre- to centimetre-thick shear zone, rich in blue or green Amp. The mineral association is: Omp (35%), Grt (25%), Gln (20%) Zo (10%), white mica (5%) and Qz (5%).